A. Technical Field
Fabrication of small circuits and circuit elements, e.g., large-scale integrated circuits (LSI) is realized through the one or more pattern delineation steps. In accordance with prevalent practice at this time, use is made of discrete masks which when finally processed, consist of apertured chromium patterns supported on glass substrates. Typically a set of six or more such masks are required for semiconductor circuit fabrication. They are utilized sequentially for replicating patterns in sensitive supported (resist) material on the semiconductor, following which the replicated pattern is utilized to define areas to be etched, plated, implanted, or otherwise processed. There is a growing technology which involves electron beam delineation to produce such masks with design rules of a few micrometers or less. Replication is generally accomplished with near u.v. light.
The expectation that economic and other considerations may lead to smaller design rules has focused attention on inherent limitations in presently used mask technology. Standing waves, interference and other limitations relating to wavelength have led to studies directed to the use of effectively shorter wavelength replicating radiation such as X-ray, electron flood, and short wavelength u.v. Anticipated yield loss due to registration difficulty with diminishing design rules is leading to evolution of a "maskless" technology known as "direct processing". In this technology, the primary pattern delineation is in resist layers made an intimate part of the device at each stage in fabrication. All such fine-line programs are dependent upon introduction of suitable resists.
B. History
A resist composition now in use in the fabrication of e-beam generated master masks is based on an addition polymer of glycidyl methacrylate (GMA) and ethyl acrylate (EA). Compositions in this category may be so formulated as to exhibit a useful sensitivity of 10.sup.-6 C/cm.sup.2 (Coulombs per square centimeter) or better as well as a rubbery consistency assuring excellent adhesion during the wet processing of underlying metal layers of which the final hard copy mask is constituted (see, J. P. Ballantyne, J. Vac. Sci. Technol., 12, 1257 (1975).
Materials exemplified by GMA-EA while of appropriate properties for fine-line delineation do not have optimal properties for all types of contemplated processing. In particular, dry processing, e.g., ion beam etching, ion implantation, sputter etching, etc., may result in degradation of the resist in the now-developed pattern, resulting in inadequate discrimination between bared and protected surface regions.
A category of negative resists still primarily dependent upon radiation-induced epoxy crosslinking depends upon inclusion of resonant moieties. One such material produced by copolymerization of GMA and styrene is disclosed in J. H. Rai and L. T. Shepherd, ACS Organic Coatings and Plastics Preprints, 35 (2), 252 (1975). Inclusion of aryl rings results in some decrease in sensitivity to delineating radiation. A related category of materials depends upon halogenation to recoup some significant part of the sensitivity lost by introduction of resonant bonding (see Feit-Thompson Case 12-16, U.S. application Ser. No. 812,231, filed July 1, 1977, now U.S. Pat. No. 4,130,424). Such resists show improved stability to the depolymerizing effect of many forms of dry processing while retaining sufficient delineation sensitivity to be candidates for production line use.
Recognizing the inherent stability of resonant structures others have utilized variations on such compositions in the formulation of direct processing resists. T. L. Brewer, Technical Papers, Regional Technical Conf., SPE, October 1973, p. 138 describes such a resist which in its exemplary form is simply a homopolymer of unsubstituted styrene.
Depending upon absorption characteristics, all such resists are usefully employed with electron beam as well as with X-ray and other short wavelength electromagnetic radiation.